1. Field of the Invention
This invention relates to a plasma source apparatus and method by which to produce a high power density gaseous plasma discharge within a nearly all-metal, fluid cooled vacuum chamber by means of an inductively coupled RF power means. The invention is particularly useful for generating a high charged particle density source of ions and electrons with either chemically inert or reactive working gases for processing materials.
2. Brief Description of the Prior Art
Numerous forms of inductively coupled plasma (ICP) sources or transformer coupled plasma (TCP) sources have been described along with their applications to materials processing. These devices make use of at least one induction coil element disposed in close proximity to, around or within a vacuum chamber that is excited with RF power. Electromagnetic fields about the induction coils sustain a gas plasma discharge within the vacuum apparatus and induce RF electron drift currents within the plasma. In addition to the RF induction coils, auxiliary DC or AC magnetic fields and Faraday shields may also be imposed on the ICP or TCP apparatus so as to enhance RF heating or spatial properties of the gas plasma discharge. References have disclosed unique design embodiments of the induction coils, the application of Faraday shields to reduce electrostatic coupling from high voltages on the coils, RF coupling and impedance matching techniques to the ICP source, and unique embodiments that improve the RF electron heating and spatial uniformity of the plasma.
In those instances where the induction coil is not immersed within the vacuum, the RF fields are coupled to the plasma discharge through a ridged dielectric vacuum wall. The plasma discharge body formed by various ICP sources can be closely coupled to a materials processing vacuum region to facilitate surface treatment, material etching, sputter deposition, chemical vapor deposition, and other related vacuum based operations. In other applications it is desirable to remotely couple ICP sources to process vacuum systems in order to generate heated, reactive gaseous species which then flow into the materials processing vacuum region or system to enhance etching or deposition processes and to facilitate chamber and vacuum component surface cleaning. This latter application may require a high plasma density state that is commonly associated with an ICP source in the presence of high gas flows several hundreds of standard cubic centimeters per minute (sccm) and with plasma discharge pressure vacuum pressures greater than several hundreds of milliTorr.
The following references are indicative of inductively coupled plasma sources used in materials processing applications:
U.S. Pat. No. 4,065,369, Ogawa et al., Dec. 27, 1977
U.S. Pat. No. 4,368,092, Steinberg et al., Jan. 11, 1983
U.S. Pat. No. 4,431,898, Reignberg et al., Feb. 14, 1984
U.S. Pat. No. 4,948,458, Ogle, Aug. 14, 1990
U.S. Pat. No. 5,008,593, Schilie et al., Apr. 16, 1991
U.S. Pat. No. 5,280,154, Cuomo et al., Jan. 18, 1994
U.S. Pat. No. 5,397,962, Moslehi, Mar. 14, 1995
U.S. Pat. No. 5,534,231, Keller et al., Jul. 9, 1996
There are several shortcomings of the prior art methods for producing inductively coupled plasma for many applications. In particular, these sources which use non-conductive dielectric (i.e. fused-quartz, glass, alumina, sapphire, aluminum nitride, or boron nitride) walls to separate the induction coil(s) from the plasma discharge body can limit range of the power density (Watts per cubic centimeter) disposed through the dielectric wall and into the process plasma discharge. These dielectric vacuum wall materials pass the RF fields into the vacuum plasma discharge. They are then selected and applied on the basis of their chemical compatibility with the reactive plasma discharge byproducts, thermal-mechanical resilience, and UV transmission properties. However, at higher pressures and power densities, the restrictions of collisional, ambipolar diffusion of ions and electrons results in a spatial constriction of the plasma discharge within the vacuum chamber, and often in close proximity to the induction coil current carrying elements. In such cases the heated plasma discharge gases readily transfer thermal energy to the wall in close proximity to the induction coil as disposed about the plasma source. The localized thermal flux from the plasma gases can be so high and localized that the plasma discharge may damage the dielectric materials by means of thermally enhanced chemical erosion, thermally induced mechanical stresses, or even softening and melting as in the case of fused quartz or glass. Moreover, the spatial constriction of the diffusive plasma body can result in a lower coupling efficiency from the primary induction coils to the gas plasma discharge.
In order to accommodate these dielectric materials under high power density conditions, special fluid cooling measures such as forced air cooling or circulating liquid coolants in direct or near contact to the dielectric vacuum wall is required to extract waste heat from the plasma apparatus. Exemplary of this approach are Schile et al. in U.S. Pat. No. 5,008,593, Holber et al. in U.S. Pat. No. 5,568,015, and Shang et al. in U.S. Pat. No. 5,892,328. The inclusion of fluid cooling measures directly on the dielectric vacuum plasma discharge walls can make the design of the source complicated and expensive and can reduce the coupling efficiency of RF or microwave power to the plasma. Also, direct water cooling of the dielectric wall can incur high operational risks because air or water can readily leak into the vacuum based process should the dielectric wall be cracked or damaged when mechanically or thermally shocked or when under cooled.
In some cases, workers can avoid the problems with the high power density inductively coupled state at high pressures and power by operating ICP plasma discharge apparatus in a substantially weaker plasma density, capacitively coupled state. In this lower plasma density state, the plasma discharge is driven by sheath electron heating through the spatially distributed quasi-static electric fields from the RF voltage on the induction coil rather than through induced electric fields associated with the RF currents on the induction coil. Unfortunately, the capacitively coupled state of conventional ICP sources does not support the high excitation, ionization, and molecular dissociation levels as desired from the inductively coupled state of an ICP source.
Most existing ICP and TCP sources are limited in their scale, power range and pressure range of operation due to the following:
1. extensive use of dielectric vacuum wall materials and their common thermal mechanical constraints when used in high power density, chemically reactive and ultraviolet light emitting plasma applications;
2. low coupling efficiency within non-immersed coil ICP source designs ( less than 0.5 typically) due to spatial design constraints between induction coils and this plasma discharge body;
3. constraints in cooling dielectric vacuum walls, vacuum seals, vacuum power feedthroughs, and induction coil elements in high power density applications; and
4. RF quasi-electrostatic fields on the coil and within the plasma as driven by the induction coil elements and the need for Faraday shields and fluid cooling means with high AC voltage standoff ratings.
Relevant to the background of this invention is an alternative means of producing an inductively coupled plasma discharge using a ferrite transformer core such that the conductive gas plasma discharge works as a single turn secondary winding about the ferrite core. This method has been described by Anderson in U.S. Pat. No. 3,500,118, U.S. Pat. No. 3,987,334, and U.S. Pat. No. 4,180,763 for lighting applications and by Reinberg et al. in U.S. Pat. No. 4,431,898 for plasma resist strip applications. These references describe how at least one ferrite core disposed about a closed path, topologically toroidal shaped dielectric vacuum chamber or chamber embodiment may be driven at RF frequencies to induce a topologically solenoidal shaped electric field and, thereby, toroidal a gas plasma discharge within the chamber. In this apparatus the plasma discharge acts as a single turn secondary current linked ferrite core with good coupling efficiency. The electron drift currents induced in the plasma flow along the closed path of the topologically toroidal volume defined by or in communication with a vacuum chamber. However, the materials used in the examples of such devices are dielectrics (i.e. fused-quartz, pyrex, nonconducive glass, alumina, sapphire, boron nitride, ceramic or heat resistant plastic materials). For the thermal management reasons noted above, these dielectric materials and vacuum walls are not well suited to applications employing very high power densities when flowing chemically reactive process gases (e.g. fluorine-bearing gases) through a vacuum chamber or other processing chamber.
Also the discussions from existing references provide no teaching as to how apparatuses using torodial shaped, transformer coupled means may be adapted to facilitate processes that require effective thermal management of its elements. For example if one were to alter the invention described by Reinberg, et al. in U.S. Pat. No. 4,431,898 by using a water-cooled conductive metal for the xe2x80x9ctubing means to define a continuous open path about an openingxe2x80x9d through a plasma chamber, the device would not work. Rather, one would couple currents onto the conductive metal body and no plasma would form within the vacuum chamber as described by Reinberg et al.
It is the objective of this invention to provide an inductively coupled plasma source apparatus that may be applied to material processing applications and that does not make principal use of primary induction coil elements that are either (1) separated from the plasma discharge by a dielectric vacuum wall or (2) are immersed within the vacuum plasma discharge. It is also the objective of this invention to have an inductively coupled plasma source that has a high coupling coefficient between the primary induction coils and the induced secondary plasma currents within a plasma discharge body that is predominantly surrounded by vacuum boundaries that can be directly cooled with a fluid. These vacuum boundaries can be selected and designed so as to be chemically compatible with reactive process gases of a high power density plasma environment. It is further the objective of this invention to provide a scalable inductively coupled plasma source apparatus which may be well cooled so as to be reliably and continuously operated over a broad range of vacuum pressure, power densities, chemistries and RF frequencies.
There is provided by this invention an inductively coupled plasma source apparatus and method. The apparatus utilizes at least one ferrite transformer core to induce closed path electron RF electron drift currents within a plasma body surrounded by a fluid-cooled hollow metal vacuum chamber. This hollow metal vacuum chamber is coupled to and electrically insulated from a metal vacuum process chamber by means of at least two orifices and dielectric gaps which are well shielded from direct exposure to the plasma body. By this means a gaseous plasma discharge is driven within an all metal hollow vacuum chamber core in such manner that the closed path RF currents are extended into said metal vacuum process chamber to form a diffusive plasma within said metal vacuum process chamber. Electrons, photons and excited gaseous species are generated within the metal hollow chamber and process chamber to serve a wide variety of material, surface and gas processing applications. This plasma source apparatus is particularly useful for generating a high chargedparticle density source of ions, electrons, and chemically active species to serve various plasma related processes that may require high power densities.
The hollow metal vacuum chamber is fluid cooled in order to remove waste heat from the plasma discharge-based process. An electrostatic element within the hollow metal vacuum chamber or vacuum process chamber is provided which is capable of forming an electric field of sufficient strength to electrically break down the gas and ignite the plasma.
This invention teaches grouping or ganging together several hollow metal vacuum chamber assemblies about a single vacuum process chamber to achieve desired plasma properties related to power, plasma density, spatial plasma formation and distribution for scaling to large areas, and excited plasma chemistry and spatial dispersion. The design and control of such plasma properties can be necessary to achieve various material process objectives.